Recently, in a rapidly-developing mobile communication system, for an improvement of efficiency in use of radio wave frequencies, various multiple access systems have been proposed that allow a plurality of users to share a radio transmission path and conduct communications simultaneously, and are now partially in practical use.
For example, PHS (Personal Handyphone System) generally employs, as a method of splitting a transmission channel, TDMA (Time Division Multiple Access) that divides a radio channel with respect to time.
The TDMA system transmits respective digitized signals of users by radio waves at different frequencies in separate and certain time periods (time slots), and separates respective signals of the users by frequency filters as well as time synchronization between a base station and respective mobile terminal devices of the users.
Further, for a further improvement of efficiency in use of radio wave frequencies, SDMA (Space Division Multiple Access) system in which a single time slot of a single frequency is spatially divided to transmit data of a plurality of users has been proposed.
In both of the TDMA system and the SDMA system, a time period (1 frame) using one frequency is divided into several time slots so as to allow each user to use different time slot. Accordingly, time-position control (burst synchronization control) for accurately transmitting signals to allocated time slots in order to avoid overlapping of the signal with other user's transmission is required at the time of transmission.
In the PHS, synchronous position detection utilizing what is called correlation synchronization has conventionally been known as a method of locating a synchronous position of a signal received from a personal station (hereinafter, referred to as “PS”) in a cell station (hereinafter, referred to as “CS”).
According to the synchronous position detection utilizing the correlation synchronization, a time-position at which a correlation value attains a peak is specified as the synchronous position in the following manner. Specifically, in view of the fact that a reception signal from a PS includes in each frame a reference signal (a unique word signal: UW signal) section consisting of an already-known bit string common to all users, a CS calculates correlation of an already-known waveform of the reference signal stored in a memory in advance and a waveform section cut out while shifting the waveform of a reception signal from the PS having a prescribed length.
The above-described correlation synchronization, however, is based on the premise that the CS receives a signal from a single PS, and it is not adapted to a case in which the CS receives signals of the same frequency from two PSs in an overlapped manner.
In the SDMA in particular, a single time slot of the same frequency is spatially divided and resultant channels are allocated to a plurality of PSs. Therefore, it is highly likely that two signals of the same frequency arrive at the CS in an overlapped manner.
If synchronous positions of the reception signals from the PSs of the plurality of users coincide with one another, reference signal sections of the reception signals overlap with one another and the users cannot be identified or separated from one another, resulting in interference among the users.
In any systems other than the SDMA system as well, a CS in a certain cell could receive from a desired PS a signal of a certain frequency on which an unnecessary interference wave of the same frequency of another PS connected to a CS in a different cell overlaps.
The aforementioned event is likely to occur in a traffic-busy region like urban area in which CSs are densely placed. Since the synchronous position of the signal received from the desired PS cannot be specified, interference also occurs between users in different cells, resulting in deterioration in communication quality.
In order to solve the problem above, control such as forming a directivity pattern in accordance with a direction of the PS by means of an adaptive array apparatus or slightly shifting a signal transmission time for each of the plurality of PSs establishing space division multiple access has been carried out so far in the CS of the SDMA system.
In addition, recently, Japanese Patent Laying-Open No. 2001-231072, for example, has proposed a measure to prevent mutual interference among a plurality of users by appropriately separating and extracting communication data of each mobile station by designating and using different reference signal for each PS in the CS.
Here, an operation for controlling allocation of a reference signal performed by the conventional CS on each PS will be described with reference to FIG. 9.
FIG. 9 is a flowchart illustrating an operation performed by a control unit within the CS when a link channel establishment request is received from the PS for initiating a call or data communication.
It is noted that, as the precondition for the operation in FIG. 9, a reference signal is stored as unique word information in advance in a unique word storage unit within the CS in a fixed manner.
The unique word information represents information obtained by associating a unique word value with information indicating a status that the unique word has been allocated. The unique word takes a 16-bit value such as “0011110101000110” expressed as a binary number, or a 32-bit value. In addition, the unique word takes a value specifying a PS such as user 1, user 2 and so on as a value indicating a status that the unique word has been allocated, or a value indicating that the unique word has not yet been allocated.
Referring to FIG. 9, initially, when a link channel establishment request is received from the PS (step S301) or when a link channel reestablishment request is received (step S302), a control unit searches for a channel available for allocation to the PS (step S303).
If there is no channel available for allocation (step S304), control such that a link channel allocation rejection notification is transmitted to the PS is exerted (step S308).
On the other hand, if there is a channel available for allocation (step S304), referring to the unique word held in the unique word storage unit, the control unit determines that an unallocated unique word is to be allocated to the PS and updates the unique word information so as to associate the unique word with the PS (step S305).
In addition, the control unit transmits to the PS a link channel allocation notification including notification of the determined, one unique word (step S306).
Furthermore, the control unit provides a value of the unique word included in the notification, to a reference signal generation unit in a user processing unit that carries out processing adapted to the PS (step S307).
Thus, the conventional CS can determine the reference signal based on the unique word value and appropriately direct, based on the reference signal and a reception signal, a directivity pattern to the PS. Accordingly, interference with other PSs can be avoided and communication can be conducted with the PS in the SDMA system while communication quality is retained.
The conventional CSs heretofore discussed are all techniques specific to SDMA to prevent mutual interference between a plurality of users in a cell and thereby retain certain communication quality.
There still remains a problem, however, that the conventional CSs do not sufficiently address the problem of interference with users in another cell.
For example, as shown in FIG. 9, even when the CS designates different unique words for respective PSs in the cell, a unique word designated for one PS is not the one that is designated in consideration of any PSs in other cells. Therefore, in a certain time slot, the designated unique word could be identical to a unique word designated for a PS in another cell. In this case, although interference with other PSs in the same cell can be avoided, interference with any PSs in other cells could still occur.
It is noted that the above-described situation is not peculiar to the SDMA system and could occur as well to the TDMA system.
An object of the present invention is therefore to provide a radio cell station apparatus, a reference signal allocation method and a reference signal allocation program that can avoid mutual interference between PSs in adjacent cells and thereby prevent deterioration in communication quality.